1. Field of the Invention
This invention relates to a wireless communication device and a method of determining transmission rates of a wireless communication device, especially to a wireless communication device and its associated method that adaptively select transmission rate according to the conditions of the communication environment.
2. Description of Related Art
In a wireless communication system, a system is typically provided with a plurality of transmission rates. The selection of transmission rate depends on a variety of conditions. When a receiving end is far away from the transmitting end, the transmission rate must be low so that it is easier for the receiving end to receive data packets. On the other hand, when the receiving end is close to the transmitting end but there is much environmental interference or another wireless device is competing for the access to this channel, a high transmission rate is better because a high transmission rate shortens the transmission time of a data packet so that environmental interference and interference or collision from the wireless signals of other wireless devices are less likely to happen. Therefore, an appropriate transmission rate adjusting method is critical to a wireless transmission system.
In a wireless communication system, an appropriate transmission rate, sufficient transmission power, and good signal quality are essential for a data packet to be correctly transmitted from the transmitting end to the receiving end. If the transmission power and the signal quality do not change, the transmission rate must be properly adjusted to meet the requirement at the receiving end so data packets can be correctly transmitted to the receiving end. Many wireless communication systems have a retry mechanism. In the 802.11 standard, a transmission method that uses carrier-sense multiple-access, collision avoidance (CSMA/CA) with acknowledgement (ACK) is adopted. In this method the transmitting end waits for a period of time to detect whether there are other signals in the air before transmitting a data packet, and transmits the data packet if no transmission of other data packets at that time is confirmed. After the data packet is transmitted, the transmitting end will transmit the data packet again if an ACK packet from the receiving end is not received after a certain period of time, implying a failure transmission of the data packet. Reasons for the receiving end not being able to receive the data packet may be many but are basically classified into two main types. The first type is that a signal-to-noise ratio (SNR) of the data packet received at the receiving end is too low so the receiving end cannot receive the data packet correctly. The second type is that the transmitted data packet encounters either interference or collision with wireless signals from other wireless devices so the receiving end cannot receive the data packet correctly.
If it is the first case, the transmitting end must transmit the data packet at a lower transmission rate. The requirement for SNR at the receiving end can be lower due to a lower transmission rate; therefore, compared to a higher transmission rate, the data packet at a lower transmission rate is more likely to be correctly received. Another solution to the first case is to increase the transmission power, which is equivalent to increasing the SNR at the transmitting end, so the data packet which were not received correctly can then be received correctly. If it is the second case, the transmission rate should not be lowered. If the transmission rate is lowered, collision probability will be increased due to a longer transmission time in the air of the data packet.